1. Field of the Invention
The present invention relates to an automatic focusing apparatus for focusing the lens of a camera so that the image of an object to be taken is focused accurately at a predetermined position.
2. Description of the Prior Art
Various automatic focusing systems for a camera have been proposed and applied to practical still cameras. Recently, applications of the automatic focusing system have been spreading in the field of video cameras.
Among those automatic focusing systems, an active automatic focusing system is a promising one. The active automatic focusing system projects a range finding signal, such as an infrared signal or an ultrasonic signal, against an object to be taken (referred to simply an "object" hereinafter), then receives the reflected range finding signal reflected by the object, and then drives the picture taking lens system to a correct focusing position according to the range finding signal.
FIG. 1 shows the principle of an active automatic focusing system employing infrared rays as a range finding signal. Referring to FIG. 1, an infrared range finding signal projected by a projecting unit 1 travels through a condenser lens 2 and falls on an object 3 and is reflected by the object 3. Then, the reflected infrared range finding signal travels through a condenser lens 4 and falls on a light receiving element 5. The angle of incidence or the point of incidence of the reflected infrared range finding signal on the light receiving element 5 varies according to the variation of the distance between the object 3 and the projecting unit 1 and the distance between the object 3 and the light receiving element 5. An arithmetic unit 6 executes a predetermined operation according to the angle of incidence or the position of incidence of the reflected infrared range finding signal. A lens driving uit 7 drives a picture taking lens system 8 comprising a plurality of component lenses according to the result of the operation to an appropriate focusing position to complete the focusing operation.
FIG. 2 is a conceptional illustration of a conventional active automatic focusing system employing infrared rays as a range finding signal. A light emitting diode 9 which emits infrared rays (referred to as "infrared LED" hereinafter) is driven by a LED driving unit 10 so as to project range finding infrared rays modulated in a fixed frequency or intermittent range finding infrared rays to discriminate the range finding infrared rays from the ambient infrared rays. The range finding infrared rays travel through a condenser lens 11 and are reflected by an object, not shown. The reflected range finding infrared rays travel through a condenser lens 12 and are focused on a light spot on the surface of a photodiode 13 divided into two parts (referred to as "two-part PD" hereinafter). The two-part PD provides currents I.sub.1 and I.sub.2 corresponding to the position of the light spot on the surface of the two-part PD. That is, if the center of a light spot 14 is located above the partition line 16 of the two-part PD 13 as shown in FIG. 3(a), EQU I.sub.1 &gt;I.sub.2 ( 1)
if the center of a light spot 15 coincides with the partition line 16, as shown in FIG. 3(b), EQU I.sub.1 =I.sub.2 ( 2)
and if the center of a light spot 17 is located below the partition line 16 as shown in FIG. 3(c), EQU I.sub.1 &lt;I.sub.2 ( 3)
Referring again to FIG. 2, first and second signal processing units 18 and 19 converts the photoelectric currents I.sub.1 and I.sub.2 produced by the two-part PD 13 into voltages V.sub.1 and V.sub.2 which are proportional to the photocurrents I.sub.1 and I.sub.2, respectively. A comparator 20 compares the voltages V.sub.1 and V.sub.2. A decision unit 21 controls a lens driving unit 23 according to the output of the comparator 20 to move a focusing mechanism 25 for focusing a picture taking lens 22 (represented typically by a single convex lens) frontward or rearward. A photodiode moving unit 24 interlaced with the focusing mechanism 25 moves the two-part PD in either one of the directions indicated by arrows in FIG. 2 as the focusing mechanism 25 operates. The operation of the focusing mechanism 25 is stopped when the two-part PD arrives at a position where the photoelectric currents I.sub.1 and I.sub.2 are equal to each other. Thus the automatic focusing operation is achieved. In this conventional active automatic focusing system, the setting position and the mechanical displacement of the two-part PD 13 are adjusted beforehand so that the position of the focusing lens group of the picture taking lens and the position of the two-part PD are in one-to-one correspondence.
Accordingly, the decision unit 21 controls the direction of movement of the picture taking lens 22 so that the output photoelectric currents I.sub.1 and I.sub.2 of the two-part PD will become equal to each other and stops the picture taking lens at an appropriate focusing position to achieve automatic focusing.
This conventional active automaic focusing system has strong points that the range finding accuracy is influenced scarcely by the subject brightness and the subject contrast and that the employment of the infrared LED enables the directional angle to be reduced to a comparatively small angle. Thus the conventional active automatic focusing system is useful.
However, a video camera, in particular, is required to take a continuously moving object in addition to a stationary object. Therefore, the conventional active automatic focusing system needs to project infrared rays continuously while the picture taking lens is being moved to an appropriate focusing position, and hence the operation of the active automatic focusing system requires large power consumption when the object moves over a wide range (for example, changing the object from a near object to a distance object takes several seconds). Ordinarily, the active automatic focusing system continuously requires an electric current as high as 100 to 300 mA during the focusing operation, which is a significant problem with a focusing system for a portable video camera which is operated by power supplied from a battery unit. Furthermore, since the photodiode moving unit needs to be coupled with the picture taking lens focusing mechanism, the disposition of the photodiode is restricted, and hence many restrictions are placed on the design of the mechanism of the picture taking lens.
The dynamic range of signals to be processed by an active automatic focusing system will be examined hereunder. Generally, the quantity of light that enters the photodetector is proportional to the quantity of light emitted by the infrared LED, proportional to the infrared reflectivity of the object and inversely proportional to the square of the distance L between the object and the photodetector. If the infrared reflectivity is 10 to 100% and the range of distance measurement is 1 to 20 m, the dynamic range of the signal to be processed is as wide as 1:4000. Accordingly, the performance of the active automatic focusing system is dependent on the SN ratios (signal-to-noise ratios) of the photodetector and the initial amplifier and the linearlity of the signal processing system over such a wide dynamic range against the scatter of the circuit elements in quantity, temperature variation and DC offset. However, such a conventional active automatic focusing system is unable to achieve distance measurement at a satisfactory accuracy.